This invention relates generally to waste disposal landfills and is particularly concerned with abatement of leachate formation in a waste disposal landfill. More specifically, leachate abatement is accomplished by a ventilation layer with means to induce airflow through the ventilation layer so as to evaporate water which may have intruded into the landfill and to remove the water from the landfill.
There exists substantial concerns about the possibility and potential adverse consequences of long-term leachate formation within hazardous waste disposal landfills. Indeed, governmental regulations have recently mandated positive means for inhibiting formation of and removing leachate from hazardous waste disposal landfills during their active lives. For example, sloping the bottom liner of the landfill, providing a collection drainage blanket and collection pipe, and specific treatment of leachate are mandated.
Notwithstanding stringent requirements for both the bottom liner to preclude exfiltration of any residual leachate and providing a highly impermeable and properly sloped cover for the landfill, there is still concern that some small portion of the local precipitation will find its way through the final cover and over a period of time generate appreciable leachate which in turn may escape the landfill. If nothing else, slowly accumulating leachate could fill and ultimately spill from the landfill (so called bathtub effect).
The currently specified remedy for this is to periodically pump such leachate as may accumulate from the collection system for an indefinite period of time. This is both expensive and bothersome, and there will be the temptation to delay pumping until a "worthwhile" amount of leachate has accumulated. This will increase the risk that some of the leachate will exfiltrate in an uncontrolled and undefined amount.
The present invention is directed to providing a new and improved waste disposal landfill with a particular improvement in abating leachate formation. Through such abatement, the concerns expressed above will be substantially lessened, if not entirely eliminated. In addition to its objective attributes in minimizing or eliminating leachate formation, the invention possesses the advantage of being a purely mechanical system comprised of relatively inexpensive, proven, and readily available materials and apparatus. Moreover, it can be easily inspected and repaired if necessary. It is also redundant and can be modified without the necessity of exposing the in situ waste. The system can be installed to supplement the final cover of a waste disposal landfill over a wide range of suspect or uncontrolled hazardous wastefills thereby to provide assured long-term abatement of surface infiltration and consequent leachate generation and possible migration from the landfill.
The invention arises in part through applicant's recognition that at most, if not all, current or potentially available landfill sites, certain natural conditions or circumstances prevail which can be used to advantage in substantially minimizing or entirely eliminating leachate formation. Included among these conditions and circumstances are the following: (A) generally less than one inch of annual rainfall is likely to infiltrate through a proper final cover of a waste disposal landfill; (B) there are extended periods of time during which the ambient atmospheric relative humidity is below 100%; (C) during a preponderance of the year, the dew point temperature of ambient air is well below that of the prevailing temperature in the upper portion of the waste landfill (i.e. immediately below the final clay cover); and (D) during a preponderance of the year there are appreciable natural wind currents.
Recognizing and utilizing these naturally occurring environmental phenomena, applicant has invented a waste disposal landfill in which the ambient relatively dry atmospheric air (relative humidity less than 100%) is forced by natural wind forces through a porous ventilating layer interposed between the top of the waste and the underside of the final cover. The ventilating layer will intercept and temporarily retain the limited volumes of moisture which may percolate from the overlying cover. The concomitant air movement through the layer will tend to evaporate, convey and remove any free moisture which it encounters as well as to diffuse through and thereby reduce the overall humidity of air which it may encounter in the layer.
In one embodiment of the invention, air enters through intake pipes located at the periphery of the covered landfill and exits from one or more centrally located exhaust pipes. Both are provided with self-aligning airscoops. Intake pipes are provided with self-aligning airscoops to face upwind and to provide a partial positive pressure; exhaust pipes with an analogous self-aligning airscoop facing downwind to provide a partial negative pressure, thereby to facilitate movement of ambient air through the ventilating blanket layer. The number and location of pipes are chosen to provide adequate ventilation below the entire area of the final cover.
It is possible to utilize the invention with either a fully enclosed landfill (i.e., one having an impervious bottom liner) or a partially enclosed landfill (i.e. without impervious bottom). In the latter case, a wick and blanket capillary system may be used to provide a blanket portion coextensive with the ventilation layer and with a wick portion extending through the waste material to a level below that of the prevailing water table. The capillary action will be effective to induce a capillary flow of any accumulated water upwardly within the landfill to the ventilating layer where it is removed in the manner explained above. With the partially enclosed landfill being in hydraulic communication with the surrounding water table, the capillary action creates a hydraulic gradient from the water table toward the interior of the reservoir so as to thereby impede the recharge of ground water which is being removed from below the waste deposit. The rate is sufficient to maintain the interior free water level below that of the surrounding water table and the hydraulic gradient thus created inhibits the outward excursion of leachate which might otherwise occur.
Several embodiments of the invention are disclosed herein in order to illustrate its principles.